Arron Banks, the co-founder of the Leave.EU campaign

Leave.EU has been fined £70,000 and its chief officer has been referred to the Metropolitan police after the Electoral Commission found it had breached multiple counts of electoral law during the referendum to leave the European Union.

The investigation found that Leave.EU, which was co-founded by Arron Banks, unlawfully exceeded its statutory spending limit by at least 10% and delivered incomplete and inaccurate spending and transaction returns.

The group campaign chief, Liz Bilney, faces a police investigation. The commission said it had reasonable grounds to suspect that she “knowingly or recklessly signed a false declaration accompanying the Leave.EU referendum spending return”.

The commission said the “unlawful overspend” was at least £77,380 but “may well have been considerably higher”.

It said services the group received from the US campaign strategists Goddard Gunster were not included in the spending return “despite a proportion of them having been used during Leave.EU’s referendum campaign”.

The commission also found that Leave.EU inaccurately reported three loans it had received.

It added: “This included a lack…